One avenue of investigation is already closed off to forensic officials working the Boston Marathon bombing case due to efforts dating back decades by the National Rifle Association and gun manufacturers.

The FBI said Tuesday that gunpowder, along with pieces of metal and ball bearings, were packed into at least one pressure cooker and another device to make the crude bombs that killed three people—including an 8-year-old boy—and wounded more than 170 more during the Boston Marathon Monday.

But a crucial piece of evidence called a taggant that could be used to trace the gunpowder used in the bombs to a buyer at a point of sale is not available to investigators.

“If you had a good taggant this would be a good thing for this kind of crime. It could help identify the point of manufacturer, and chain of custody,” Bob Morhard, an explosives consultant and chief executive officer of Zukovich, Morhard & Wade, LLC., in Pennsylvania, who has traced explosives and detonators in use in the United States and Saudi Arabia, told msnbc.com. “The problem is nobody wants to know what the material is.”

Explosives manufacturers are required to place tracing elements known as identification taggants only in plastic explosives but not in gunpowder, thanks to lobbying efforts by the NRA and large gun manufacturing groups.

NRA officials at the group’s headquarters in Fairfax, Virginia declined to respond to calls and emails from msnbc.com requesting comment.

“They are concerned about tort liability,” Morhard added to msnbc.com, referring to manufacturers worried about being sued over the improper use of their ammunition or explosives. Worries about the cost of adding taggants to gunpowder were also raised by the Institute of Makers of Explosives. NRA officials seem more concerned about government use of technology to trace either firearms or the gunpowder used to make ammunition. Fear of government use of tracking technologies is also echoed online.

“These taggants would allow the police to identify the maker and even the lot of the ammo by the taggant,” posted blogger dfariswheel online in January in a closed gun-forum called AR15.com, a longstanding group named for the same type of military-style, semi-automatic rifle used in both the Newtown grade school and Aurora movie theater mass shootings.

In the past, the NRA has argued that taggants could affect the trajectory of bullets and would also be a de facto form of weapons registration, reported the Los Angeles Times in 1995.

Yet, one of the NRA’s own “Fact Sheets” from the 1990s on the website of its lobbying wing expresses reservations about taggants but still indicates that they could work.

“Identification taggants are microscopically color-coded particles that, if added to explosives or gun powders during their manufacturing, might facilitate tracing those products after a bombing back to the manufacturer,” reads the 1999 post “Taggants and Gun Powers” by the NRA’s Institute of Legislative Action. “Then, through the use of mandatory distribution records, tracing would continue through wholesaler and dealer levels to an original purchaser or point of theft.”

The same NRA, however, has twice deployed its lobbyists to block the mandated use of identification taggants by gunpowder manufacturers.

The first time came more than thirty years ago, after a wave of bombings in the 1970s mainly by the radical left Weather Underground and Puerto Rican nationalist groups.

A congressional study in 1980 found: “Identification taggants would facilitate the investigation of almost all significant criminal bombings in which commercial explosives were used.”

But the NRA successfully lobbied to have black and smokeless gunpowders exempted from the explosives required to include taggant markers. Members of Congress—including then-New York Rep. Charles Schumer– tried and failed again after the 1993 New York City truck bombing of the World Trade Center. The Clinton administration renewed the call for legislation requiring identifying taggants right after the 1995 Oklahoma City bombing, whose 18th anniversary is Friday.

The NRA backed a National Research Council committee in 1998 to examine taggant technologies, later claiming the committee found them to be “unfeasible and of uncertain value.”

In fact, the committee concluded: “Identification taggants and an associated record-keeping system could be of further assistance in tracking down bombers in cases where basic forensic techniques fail.” The committee added that “additional research on these systems is needed to determine whether they are safe and effective.”

Little or no known public research has been done on the matter since, as the NRA gained more national influence in the 2000s during the administration led by President George W. Bush.

“It was explained that taggants would alter the powder in unsafe ways and that no military or police organization would allow it in their ammo, and that the unknown and unsafe taggants effects would likely cause explosive accidents,” continued dfariswheel in his January post on AR15.com.

Although this online forum is closed to unregistered users, individual threads are still partially visible via Google, which is how msnbc.com reached this thread’s unique URL address.

“That’s really a stretch,” said Morhard. Some taggants are themselves explosive and others are toxic, even carcinogenic, he added, but the risks are concentrated among employees storing and inserting the taggants into explosive products.